Toxic gas, such as hydrogen sulfide gas, can be deadly even at low concentrations. Generally, when one is exposed to such a toxic gas, it is imperative to seek medical attention relatively quickly. Accordingly, in many industrial situations, it is very important to be able to detect toxic gas in very low concentrations as soon as possible when a leak occurs, even in the most challenging and remote conditions. Moreover, it is important that a toxic gas sensor be ready to perform its function even when the occurrence of a toxic gas leak is extremely rare.
Health and safety standards in many countries have been slowly decreasing the acceptable exposure levels as sensor response times and overall stability of sensing elements have improved. For example, in the United States, the Occupational Safety and Health Administration (OSHA) provides an acceptable concentration limit for exposure to hydrogen sulfide at 20 parts per million (ppm) for an 8-hour period, with the maximum peak exposure at 50 ppm for 10 minutes.
An important goal of any fixed-location toxic gas detector is to safeguard workers and the public by warning of the presence of hazardous levels of such toxic gas in the proximity of the sensor. Electrochemical and Metal Oxide Semiconductor (MOS) cells have, for many years, been field-proven toxic gas sensing technologies. MOS-based sensors have a long life compared to electrochemical sensors and continue to operate in wide ranging temperatures, particularly high temperatures, as well as in extremely dry conditions.
In some implementations, a toxic gas sensor may be constructed as a sandwich of a platinum heater element, an insulation medium and gas-sensitive resistive film. In other implementations, a toxic gas sensor, such as a hydrogen sulfide sensor, may be constructed as a bead having a heater disposed therein and a lead wire running through the bead. The bead is formed of a gas-sensitive semiconductor. This gas-sensitive material will employ traditional metal oxide semiconductor materials or metal oxide semiconductor materials that are enhanced at the nano-level to dramatically improve performance. During operation, when a toxic gas comes into contact with the gas-sensitive material, there is a measurable change in the electrical conductivity of the sensor. These changes are typically amplified using electronics in a detector device.
This type of sensor typically utilizes the polycrystalline structure of the sensing material (semiconductor metal oxide) and the existence of the negatively charged surface oxygen species, which controls the height of the Schottky barrier and the electrical resistance of the material. When the sensor is exposed to certain reducing gases, the surface oxygen will be consumed, reducing the Schottky barrier, and the resistance, which is the sensing signal.
In order to ensure that a metal oxide semiconductor toxic gas sensor is able to provide a viable signal, it is important for diagnostics to be able to test, or otherwise determine if the sensor has become damaged, or otherwise deteriorated.